1. Field of the Invention
The present invention relates to a duobinary optical transmission apparatus using a duobinary optical transmission technique.
2. Description of the Related Art
In general, a Dense Wavelength Division Multiplexing (DWDM) optical transmission system transmits an optical signal using a single optical fiber in such a way that it enhances transmission efficiency. The optical signal is comprised of a plurality of channels having different wavelengths. In addition, the DWDM optical transmission system has been widely used for a super high-speed Internet network, which has rapidly increasing data transfer quantity, because it transmits optical signals irrespective of a transfer rate. Recently, systems for transmitting more than 100 channels using a single optical fiber using such a DWDM optical transmission method have been commercially produced. Moreover, a new system for simultaneously transmitting more than 200 channels each, which have a transfer rate of 40 Gb/s, to accomplish a transfer rate of more than 10 Tbps is under development.
Such a newly developed system accommodates rapidly increasing data traffic as well as transfer requests for high-speed data of more than 40 Gbps. However, a conventional optical intensity modulation method using an Non-Return to Zero (NRZ) method has a limitations, for example, in increasing the transfer quantity because an abrupt interference and distortion between channels occurs in a prescribed zone less than a channel interval of 50 GHz. Further, DC frequency components of a conventional binary NRZ transmission signal and high-frequency components spreading in a modulation procedure cause nonlinear characteristics and dispersion, while the DC and high-frequency components are propagated in an optical fiber medium. This, in turn, limits the transmission distance at a high-speed transfer rate over 10 Gbps.
In recent times, optical duobinary techniques have been intensively researched to find a new optical transmission technique for obviating the transmission distance limitation caused by chromatic dispersion. The optical duobinary technique has advantages in reducing the width of the transmission spectrum much more than a general binary transmission method. The transmission distance in a dispersion limitation system is inversely proportional to a square of a transmission spectrum bandwidth. That is, where the transmission spectrum bandwidth is reduced by half, the transmission distance increases by four times. Also, the carrier wave frequency is suppressed in a duobinary transmission spectrum such that limitations in output optical power caused by the Brillouin Scattering stimulated in an optical fiber are reduced.
Also, there has been a newly proposed duobinary RZ transmission method having nonlinear and dispersion characteristics superior to those of the aforementioned binary NRZ transmission method (for an intermediate-long distance transmission at a high-speed transfer rate over 10 Gbps).
FIG. 1 is a block diagram of a conventional optical transmission apparatus using a duobinary Return to Zero (RZ) transmission method.
Referring to FIG. 1, a conventional duobinary optical transmission apparatus includes a duobinary signal generator 10 and a RZ pulse generator 20 to generate a duobinary RZ signal.
The duobinary signal generator 10 includes (1) a differential precoder 11 for encoding an input two-level NRZ electric signal, (2) a drive amplifier 12 for amplifying the two-level NRZ electric signal generated from the differential precoder 11, and generating an optical modulator driving signal, (3) a LPF (Low Pass Filter) 13 for converting the amplified two-level electric signal into a three-level electric signal, and reducing a bandwidth of the three-level electric signal, (4) a laser source 14 for generating a carrier wave, and (5) a Mach-Zehnder-interferometer-type optical intensity modulator (MZ MOD) 15.
The duobinary signal generator 10 is classified according to an electrode structure of the MZ MOD 15 (generally into two kinds of generators). An X-cut type MZ MOD having a single electrode is shown in FIG. 1, and it connects its own one arm to drive amplifier 12 and LPF 13 to transmit a three-level signal to one electrode. Alternatively (not shown),a Z-cut type MZ MOD having a dual electrode connects both arms each to the drive amplifier and the LPF in such a way that a three-level electric signal is applied to each of the electrodes of the Z-cut type MZ MOD.
The RZ pulse generator 20 includes MZ MOD 21 and clock generator 22 for generating a clock signal with a period of a bit rate T.
The operation of the aforementioned conventional duobinary optical transmission apparatus is described in detail below.
The two-level N RZ data is encoded as a two-level binary signal at a differential precoder 11, and is then amplified by drive amplifier 12. The amplified two-level binary signal is applied to a LPF 13. The LPF 13 has a prescribed bandwidth corresponding to ¼ of a clock frequency of the two-level binary signal. Interference between codes is generated by excessive restriction of the bandwidth, and therefore the two-level binary signal is converted into a three-level duobinary signal because of the interference between codes. The three-level duobinary signal functions as a driving signal of the MZ MOD 15. The carrier wave generated from the laser source 14 modulates its own phase and optical intensity using the MZ MOD 15, and is thereby generated as an optical duobinary signal. The optical duobinary signal generated from the duobinary signal generator 10 is applied to a MZ MOD 21 contained in the RZ pulse generator 20 to establish a signal conversion from a NRZ signal to a RZ signal. Typically, it is well known in the art that a clock generator 22's clock signal with a period of a bit rate T is applied to an optical modulator 21 such as a MZ MOD to convert a NRZ signal applied to the MZ MOD into a RZ signal. In this way, the optical duobinary signal applied to the MZ MOD 21 is converted into the RZ signal by a MZ MOD 21 synchronized with the clock generator 22's clock signal with a period of a bit rate T.
As shown in FIGS. 2a and 2b, a duobinary RZ signal has been proposed having frequency efficiency per bit and nonlinear characteristics which are superior than those of previous NRZ and RZ signals. FIG. 2a depicts the appearance of an output signal of FIG. 1, and FIG. 2b depicts the appearance of optical spectrum characteristics of the output signal of FIG. 1.
However, such a conventional duobinary transmission technique generates a three-level electric signal using a LPF such that a difference in characteristics is generated depending on transmission quality. And, the transmission quality corresponds to the transmission characteristics of the LPF and the length of Pseudo Random Bit Sequence (PRBS). This, in turn, causes a serious problem in the overall system. Typically, a slope along which a signal's level changes from a 0-level to a 1-level is different from a slope along which a signal's level changes from the 1-level to the 0-level. However, in case of a duobinary optical transmission apparatus using a LPF, parts having different slopes are mutually summed up at one time. Consequently, increased jitters of the output waves are caused when a first signal transition from 0-level to 1-level and a second signal transition from 1-level to 0-level is performed. This jitter problem is generated in the Z-cut type or X-cut type conventional structure. The dependency of such signal patterns provides a limitation in a real optical transmission operation.